Fender Bassman 1965 AB165

[CJ]

blackface came in, not the reissue, this one was sitting in the garage for a while,  never been worked on,

 

[CJ]

inverter area

 

[CJ]

pwr

 

[CJ]

pwr trans  125RTO  606-512      puts out 337 V-rms per tap, 6.6 htr
Choke  125C1A  606-5-13      DCR 93 ohms  8.8 H at60 hz  XL 6667 Ohms at 120 Hz

Output  125A13  606513  32:1  about 8.2K to 8 Ohm spk    pri dcr taps- 46.7 and 50  L=110.5 H at 60 hz  120 vac freq flat to 60K
rez at 200 K ad 300 K  core stack  1  5/16" 

we installed some EH 6L6GC's and the amp is one of those special ones that sound really good,

 

[CJ]

under the cap can,

 

[CJ]

ajax and SK caps

 

[CJ]

here is where things get a little weird, notice the dual 100K phase inverter resistors, thought we had seen every fender circuit,

and what's with the 220K resistors?

 

[PRR]

> what's with the 220K resistors?

NFB from 6L6 plates?

 

[CJ]

yes PRR,you are right!

here is the schematic with ac and dc voltages, looks like 220K feedback resistors off the OPT,

we have neg f/b on stage 3 also instead of the typical 10pf//3.3meg attenuator,


there was some fuzz on the normal channel sine wave so we stuck some 470 pf caps across the 220K resistors and it fixed the problem,  some hi freq osc ready to almost happen, 

 

[CJ]

during the chop stick test (whacking components to check for shake rattle and roll) we noticed a change in humm, turns out that one of the hum balance 10K resistors was flopping around in the eylet, no wonder this poor amp was relegated to rusting away in purgatory,

so we fixed that and installed some electro harmonix 6L6 tubes and checked bias, let's see,

RCA put a 30 watt limit on the plates so we get safe and bump it down to 70 percent, which is 21 watts,

what is our idle current to be?

well, we have 454 volts on the plates, if P=IE, then  21 = I/454,  so I=21/454 = 46.2 ma

since we have hum balance but no bias voltage adj, we change the voltage divider resistor to 22K and bam, rock and roll,

here is a pic of the bad solder joint,CBS took over Fender in 1965, hmmm, wonder what they told the workers, speed up? 

 

[CJ]

here is the cap mod and 22k resistor,we tacked the caps so to preserve vitage value, just un tack and we do not disturb the turret board, use old school resistor for 22K on the bias pot and we are full retro,

next up is a randall orange stripe,

 

[PRR]

Note also that the whole driver-output NFB is inverting. Not non-invert like most tube amps, not that crazy injection to the long-tail tail like the 5F6a.

I think Leo peaked, and then got thinking that NFB would be better. It was maybe "better" in that it was "a new sound". (What became the sound of MusicMan.) History says the older cruder designs were more all-round musical.

There is a fad to "blackface" these too-new designs to older circuits. Often done wrong. If it's happy now, with your minimal reversible fixes, I'd leave this one alone.

 

[jensenmann]

Some Bassmans have some grounding going to the frontpanel which tends to break frequently. Make sure these solder blobs are allright.